Connell will play a central role in developing the brands of some of the agency's biggest clients, which include Heinz, Unilever, Nestle Purina, Premier Foods, Coca Cola, Sony BMG, Pernod Ricard and AOL.
Mark Artus, CEO at 1HQ, said: "We are delighted to have Karen on board. She is a big name in the industry and will be a huge asset to the company."
Connell has over 16 years' experience in brand development. Her previous clients include Coca-Cola, Cadbury Schweppes, HBOS, GlaxoSmithKline and General Motors Holden.
She is the latest high-profile signing for 1HQ, following this month's appointment of Peter Hollingsworth as managing partner of Vibrandt, the agency's branded packaging and product design arm, and last month's appointment of Mark Cox as head of employee engagement.
Artus said: "At a time when other agencies are acting cautiously, we are actually expanding and hiring some of the best and most forward thinking minds in the business.
"The reason for this is we believe the industry is currently going through a major transformation and we want to be at the forefront of this change."